1. Field of the Invention
The present invention relates to a process for manufacturing Al--Si alloys which can be used as a vehicle propeller shaft, and more particularly, to an Al--Si alloy manufacturing process which can be used as the material for use in a vehicle propeller shaft, having an excellent mechanical strength and Young's modulus and an enhanced working performance, in which a great amount of silicon (Si) added by a spray forming process is finely distributed within an aluminum (Al) matrix structure.
2. Description of Prior Art
A propeller shaft for a vehicle is a mechanical component which is installed between a transmission and a driving shaft in a vehicle, for transmitting power and has been manufactured in various forms according to purpose in use.
If a rotational speed of a propeller shaft reaches a certain value during running a vehicle, noise and fracture of the propeller shaft occurs, which is caused by a resonance between an actual oscillation of the propeller shaft and an inherent oscillation thereof. That is, since the propeller shaft is rotated by a rotational torque of an engine, a twisted oscillation occurs, and also when the center of the shaft is not coincided with the rotational shaft, a bending oscillation occurs due to the shaft being bent by a centrifugal force. When these oscillations occurring by the rotation of the propeller shaft are resonant with an inherent oscillation of the propeller shaft, the shaft is accompanied by noise and fracture phenomenon, in which case the rotational speed of the propeller shaft is called a critical rotational speed (Nc), which is expressed as the following equations (1) and (2). ##EQU1##
Here, E denotes Young's, modulus of a material (kg/mm.sup.2), .tau. denotes a specific weight (kg/mm.sup.3), l denotes the length of a shaft (mm), I denotes a secondary sectional moment (mm.sup.4), g denotes the acceleration of gravity, A denotes a sectional area of a sample, d.sub.o denotes an outer diameter of the shaft, and d.sub.i denotes an inner diameter of the shaft.
In the above equations (1) and (2), the critical rotational speed Nc can be expressed as a function of the length l of the propeller shaft, the specific Young's modulus E/.tau., the inner and outer diameters of the shaft.
Thus, since the inner and outer diameter of a shaft is chiefly determined by the mechanical strength of a material used in the shaft, it can be seen that a propeller shaft for an automobile requires a high specific modulus and mechanical strength. If a material having a high mechanical strength is used in a propeller shaft, the diameter and thickness of the propeller shaft are reduced, resulting in the lightness of the automobile and ease of the design thereof.
As can be seen from Table 1 representing the kinds and physical properties of the materials which are used as a conventional propeller shaft, the specific modulus of S45C steel is 2.73.times.10.sup.9 mm, the specific modulus of 6061 aluminum alloy is 2.65.times.10.sup.9 mm, and the specific modulus of aluminum matrix composite material such as Duralcan is 3.38.times.10.sup.9 mm. The specification (diameter .phi..times.thickness t) of the propeller shaft manufactured as S45C steel, 6061 aluminum alloy, and aluminum matrix composite material which are currently mass-produced is .phi.80 mm.times.t2 mm, .phi.120 mm.times.t2 mm, and .phi.90 mm.times.t2 mm, respectively, and the length is 1264 mm. When the length of the propeller shaft is 1264 mm, change in a critical rotational speed value according to the diameter, thickness and the specific Young's modulus of the propeller shaft calculated using the equations (1) and (2) represents a value of 8500-12500 rpm approximately.
In view of metallurgical engineering, in the case that particles having a high Young's modulus such as silicon, alumina or carbon silicate is added in aluminum matrix structure, Young's modulus of the material becomes high. In addition, as the size of the particles distributed in the matrix structure is fine, the mechanical strength increases.
However, Al--Si alloy system has a limitation in using it as a propeller shaft material for automobiles, as can be seen from the phase diagram, because the initially-crystallized Si particles are dispersed coarsely during solidification thereby causing a workability of the material to be worse, in the case that a great amount of Si is added in Al matrix structure, that is, hypereutectic Al--Si alloy is manufactured using a conventional casting process. Thus, the present inventors have studied a method for dispersing a great number of Si particles finely in the Al matrix structure. As a result, the inventors have discovered that a metal structure where a great amount of Si is finely dispersed in the Al matrix can be obtained by rapidly solidifying molten metal at a cooling rate of 10.sup.3 -10.sup.5 K/sec by a spray forming process in which droplets sprayed by a high pressure inert gas such as nitrogen flying at the state where they are not perfectly solidified, reach a substrate and are solidified completely, to thereby form a forming body, and accordingly have completed the present invention.